The present invention relates generally to current sensing devices for electrical systems, and more particularly to automatically resetable alternating current fault indicators.
With the increased use of underground electrical distribution systems wherein primary and secondary feeder cables are directly buried in the ground, the need has arisen for improved detection apparatus for determining the location of a short circuit or fault in the system cables or associated electrical components. Since the cables are buried, it is often difficult to determine the exact location of a fault, particularly where there are several distribution transformers, each serving multiple customers.
Various types of fault indicators have been constructed for detecting faults in such systems, including clamp-on type fault indicators, which clamp directly over cables in the system, and test-point type fault indicators, which are mounted over test points in the cables or associated connectors. Such fault indicators may be either of the manually reset type, wherein it is necessary that the indicator be physically reset following each fault, or of the automatically reset type, wherein a fault indication is reset upon restoration of line current. The automatically reset type is better adapted for use in underground distribution systems in that it is not necessary to gain access to the fault indicator following occurrence of a fault. Examples of such automatically reset fault indicators are found in products manufactured by E. O. Schweitzer Mfg. Co. of Mundelein, Ill., and in U.S. Pats. Nos. 3,676,740, 3,906,477 and 4,063,171 of the present inventor.
Detection of fault currents by self-resetting fault indicators is often most advantageously accomplished by means of a reed switch located within the indicator housing having contacts in close proximity to the conductor being monitored. Upon occurrence of an abnormally high fault-associated magnetic field around the conductor, the contacts close and actuate circuitry which magnetizes an internal pole piece to position to a trip position a target indicator visible from the exterior of the indicator. Upon restoration of current in the conductor another circuit is actuated to reposition the target indicator to a reset non-fault indicating position.
The present invention is directed to improvements in self-reseting fault indicators which provide for more reliable operation and improved performance. Specifically, the present invention provides electrical circuit improvements which preclude the possibility of a fault not being indicated as a result of simultaneous occurrence of trip and reset functions, and magnetic circuit improvements which preclude the possibility of a fault not being indicated as a result of the target indicator remaining fixed in a static untripped position following remagnetization of the fault indicator pole piece.
Accordingly, it is a general object of the present invention to provide a new and improved self-resetting fault indicator.
It is more specific object of the present invention to provide a new and improved self-resetting fault indicator having improved reliability and performance.